U.S. Pat. No. 8,051,827, which issued Nov. 8, 2011 and is herein incorporated by reference in its entirety, presents an overexpanded homogeneous charge spark ignition (HCSI) engine. The new HCSI engine is clean burning, while achieving significant increases in efficiency as compared to comparable existing spark ignition engines. Key features of the HCSI engine include reduced combustion temperatures and combustion pressures to reduce coolant load and friction losses and to eliminate NOx formation, and a high expansion ratio to reduce the internal energy of the exhaust gases.